Volumetric Estimates of Seismic Reflector Rotation and Convergence

Geometric attributes such as coherence and curvature are commonly used for mapping structural deformation and depositional environment. Although they are excellent at delineating a subset of seismic stratigraphic features (such as shale-dewatering polygons, injectites, collapse features, mass transport complexes, and overbank deposits) they have only limited value in imaging classic seismic stratigraphy features such as onlap, progradation and erosional truncation. We examine how the newer volumetric attributes can facilitate and quantify the use of seismic stratigraphic analysis workflows to large 3D seismic volumes.

Due to the distinct change in reflector dip and/or terminations, erosional unconformities and in particular angular unconformities are relatively easy to recognize on vertical seismic sections. Although there will often be a low-coherence anomaly where reflectors of conflicting dip intersect, these anomalies take considerable skill to interpret. Application of attributes based on the description of seismic reflection patterns can be used to map angular unconformities amongst other features. The convergence/divergence of reflections can be defined by computing the vertical derivative of apparent dip at a user-defined azimuth, and have been extended to 3D reflector convergence azimuth and magnitude estimates.

Compressive deformation and wrench faulting cause the fault blocks to rotate. The extent of rotation depends on the size, the comprising lithology and the stress levels. Fault block rotation can also control depositional processes by providing increased accommodation space in subsiding areas and erosional processes in uplifted areas. In view of this importance of the rotation of the fault blocks, we present a seismic attribute that measures the reflector rotation at every sample in the seismic data volume.

We apply these reflector convergence and rotation about the normal attributes on two different 3D seismic volumes from Alberta, Canada. The reflector convergence attribute gives the magnitude and direction of thickening and thinning of reflections on uninterpreted seismic volumes. Reflector rotation about faults is clearly evident and has a valuable application in mapping of wrench faults and subtle rotation about simpler normal and reverse faults. Such attributes would yield convincing results on datasets that have good quality.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California